The present invention relates in general to controlling aberrations in imaging devices, and, more specifically, to systems and methods for measuring and controlling an amount of aberration in electron microscopes.
Electron microscopes use a beam of accelerated electrons to illuminate a sample being tested to obtain high resolution images of the sample. Due to multiple optical elements used to direct the electron beam and to form an image of the sample, the electron microscope tends to experience certain aberrations, such as chromatic aberration, spherical aberration, etc. While these optical elements can be adjusted to address the aberration, the aberrations must be measured before they can be addressed. Measuring spherical and chromatic aberrations in a Low Energy Electron Microscope (LEEM) or Photo Electron Emission Microscope (PEEM) is a laborious process that is time-consuming and difficult to automate. In one method of measuring spherical aberration, the incident beam angle and the contrast aperture of the electron microscope are scanned in unison. As the beam angle is scanned the image shifts due to spherical aberration, defocus, and astigmatism. From systematic measurements of image shift versus beam angle, these aberrations can be measured. Another method is known as Real Space Microspot Low Energy Electron Diffraction. In this case, the sample is illuminated with a small-spot electron beam. As the sample diffracts the incident beam into a multiple of reflected beams, each with well-known angles, the images of these beams do not exactly coincide in the image plane due to spherical aberration, defocus, and astigmatism. By measuring the relative displacements of these small-spot images, these aberrations can be measured. Both of these methods require a single crystal sample to generate diffracted beams, which is not always available. To measure chromatic aberration, the energy of an incident electron beam is varied. This method requires re-alignment of the microscope for every energy setting, which is a tedious procedure. Also, when photo electrons are used, neither of these methods works. The difficulty of such testing procedures tends to limit the number of times the electron microscope is calibrated. Accordingly, there is a need to develop a method for measuring aberrations in order to take steps to correct the aberrations more routinely, without laborious calibration and measurement protocols.